1. Field of the Invention
The invention relates to the field of passive coherent population trapping (CPT) atomic clock, and more particularly to a device and a method for producing a coherent bi-color light source.
2. Description of the Related Art
As an important part of a CPT atomic clock, the physical unit includes a light source, an atomic vapor cell, and a photo detector. A laser signal produced by the light source travels through the atomic vapor cell and acts on the atomic vapor therein to generate a CPT signal which is detected by the photo detector.
A typical light source includes a laser tube and a quarter wave plate. An input end of the laser tube is connected to a current driving circuit. When in use, a direct current output by the current driving circuit is coupled with a microwave signal to drive and regulates the laser tube to output a required laser signal. The laser signal passes through the quarter wave plate and outputs a left-handed or right-handed circularly polarized light. The left-handed or right-handed circularly polarized light acts on the atomic vapor in the atomic vapor cell, and the CPT signal is detected by the photo detector.
However, when the left-handed or right-handed circularly polarized light acts on the atomic vapor in the atomic vapor cell, the majority of atoms is populated at energy levels having the minimum or maximum magnetic quantum number because of the optical pumping effect while the population of atoms at two energy levels having the magnetic quantum number of zero (which is a necessity for the transition of the atomic clock) is small. In addition, the ±1 order frequency sideband in a frequency modulated (FM) multi-chromatic laser beam produced by the existing light source primarily participates in the CPT process and other useless frequency sidebands are background light so that a very low contrast of the obtained CPT signal results, thereby affecting the stability of the output frequency of the crystal oscillator.